Take the sluice & its ring off. Apply vacuum to chamber & assure you can draw the diaphragm all the way in. Leave the vacuum, and see that no air leaks cause the release of the diaphragm.
Now put the sluice & ring on the chamber, repeat the above test.
Get a water based lube, non sticky, at Walmart. Use gobs of it. Sometimes it will just make it easier to draw in your glans.
In your photo, I don't see any lube on the glans corona, that may need more lube/comfort gel.